The stators of most conventional motors are cooled through the natural convection or forced convection of air. Some stators may be cooled using water or oil. The density of an electric current which may flow in a stator coil in the case of using the air cooling method is relatively lower than that of an electric current in the case of using the water or oil cooling method. However, the air cooling method using natural convection never requires an additional cooling device, and the air cooling method using forced convection requires the installation of only a cooling fan (blower).
The water/oil cooling method is used in relatively large capacity motors of 1000 horsepower or more, and is higher in the density of current which may flow in the stator coil in comparison with the air cooling method. But, a cooling device using the water/oil cooling method becomes very complicated.
Generally, as shown in FIG. 1, the conventional water or oil cooling method for motors is not the method of directly cooling stator coils 1 from which the most heat is emitted, but is the method of cooling a stator core 2 surrounding the coils 1, thus eliminating generated heat through heat transfer between the stator core 2 and the stator coils 1. Therefore, thanks to passages 3 through which water or oil flows, the stator yoke core is cooled.
In the conventional motor, the stator coils are surrounded by the core which has superior heat transfer ability. Thus, even though the cooling passages are provided only in a stator yoke part, the stator coils can be sufficiently cooled.
Meanwhile, a superconducting rotating machine such as a superconducting motor or generator uses superconducting coils which may generate the strong magnetic field without using the core. Since a conventional rotating machine uses coils made of copper, it is difficult to obtain a desired output power unless the core is used, and an air gap between a stator core and a rotor core is very small so as to maximize flux linkage between stator coils and rotor coils. Thus, the stator coils are disposed between slots formed in the core, thus minimizing the air gap between the stator and the rotor. However, a magnetic field concentrates on the slots formed in the core, so that the loss of an alternating current in the slots is larger than that in other parts when the magnetic field generated by the rotor rotates. Further, the slots are different in permeability from the coils, thus causing the increase in the distortion factor of the waveform of generated voltage.
In order to solve the problems of the conventional machine, the stator slot of the superconducting rotating machine is not made of the iron core but is made of a non-magnetic material such as fiber-glass reinforced plastics (FRP). Therefore, this is advantageous in that there is no loss in the slot and the waveform of generated voltage is very sinusoidal. However, since the heat conductivity of FRP is a lot smaller than that of the core, heat generated in the stator coils is not easily dissipated.
In an existing large machine such as a vessel propelling motor or turbine generator, cooling tubes 4 are disposed between the stator coils as shown in FIG. 2 or a cooling passage is provided in each coil, thus forming a water/oil cooling structure.
Most water/oil cooling structures for stators of superconducting rotating machines which have been developed until now have a configuration used in the conventional rotating machine, as shown in FIGS. 3 and 4. That is, cooling tubes 8 must be wound together with stator coils 5 in narrow slots in FRP 6. According to the bent shape of each stator coil 5, the cooling tubes 8 are bent, thus forming bent portions 17. The cooling tubes 8 of the respective portions must be welded in several places. Thus, the cooling tubes 8 are apt to become narrow or clogged, and it is very difficult to manufacture. Further, since the cooling tubes 8 are disposed in spaces in the stator slots which are to be occupied by conductors, the ratio of the stator conductors occupied in the slots is reduced, thus causing an increase in the size of the machine.
As a stator cooling structure which is different from the conventional cooling structure, U.S. Pat. No. 6,489,701 B1 has been proposed, which is shown in FIG. 5. The stator cooling structure is configured such that a slot is omitted and stator coils 9 are wound in a single layer. Cooling tubes 10 are provided on the upper and lower portions of the stator coils 9 of the single layer in such a way as to surround the stator coils 9 in a spiral form. Since the stator cooling structure has no structure for supporting electromagnetic force (torque) applied to the stator coils 9, it is suitable for an industrial motor which rotates at 1800 rpm or higher and applies relatively small electromagnetic force on the stator coils.
However, a very large electromagnetic force acts on the stator coils of a vessel propelling motor or wind turbine which rotates ten or more times slower than a general industrial motor but on which large torque acts. Thus, unless a slot for supporting the stator coils 9 is used as shown in FIG. 5, a machine may be broken or damaged.
Further, the cooling structure is an integrated structure wherein the cooling tubes 10 completely surround the stator coils. Hence, when the stator coils 9 are burn out, it is difficult to repair just them in isolation. The multiple phase coils overlap each other, so that electric insulation may easily deteriorate. Thus, the cooling structure may be applied to a high-speed and low-torque industrial motor, in which a relatively small electromagnetic force acts on the stator coils. However, the cooling structure is unsuitable for a low-speed and high-torque industrial vessel propelling motor or wind turbine, in which a very large electromagnetic force acts on the stator coils. A structure for supporting high torque is required.
Further, because of the cooling tubes 10 covering the upper and lower portions of the coils, the volume of the stator coils is increased, and thus the size of the machine is further increased. When the cooling structure is compared with other structures, the air gap between superconducting field coils and stator coils is increased. Thus, in order to ensure a flux linkage for a desired output power, a larger amount of expensive superconducting wires is required in filed coils, so that manufacturing costs are undesirably increased.